This application claims priority to Japanese Patent Application JP 2000-376008, and the disclosure of that application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to a mobile wireless terminal used for mobile communications such as a mobile telephone. Particularly, the present invention relates to a built-in antenna device disposed inside a terminal of so-called a dual band terminal which is operable at two different frequency bands.
2. Description of the Related Art
As the use of mobile telephones has spread rapidly in recent years, it has produced a tendency that a wireless communications system having single telephone circuit suffers a shortage of circuits, so that various devices and systems have been proposed to provide the necessary number of circuits by jointly using two kinds of wireless communications systems based on different frequency bands. In known such arrangements, the dual band mobile telephone terminal capable of operating in two kinds of wireless communications systems with single mobile wireless apparatus has been developed and made commercially available.
A multiplex terminal which can jointly use PDC (Personal Digital Cellular) operation on 800 MHz band and PHS (Personal Handyphone System) operation on 1.9 GHz band has been made commercially available in Japan. Another multiplex terminal capable of jointly using GSM (Global System for Mobile Communication) operation on 900 MHz band and DCS (Digital Communication System) operation on 1.8 GHz band has also been on the market in Europe and Asian countries. Moreover, another multiplex terminal which can operate on both AMPS (Advanced Mobile telephone Service) using 800 MHz band and PCS (Personal Communication Service) using 1.9 GHz band has been on sale in the United States.
As a recent trend of mobile wireless terminals for mobile communications, there are put on sale a number of terminals containing so-called built-in antenna disposed inside the terminal body. As compared with the related art antenna attached to outside a mobile wireless terminal body (so-called whip antenna), the built-in antenna has the advantage of that it is less likely to be damaged due to a fall or the like as well as additional benefits such as ease of designing.
FIG. 18 shows an example of a construction of a plate-type (micro-strip) inverted F antenna that is used as a built-in antenna for a mobile wireless terminal of the related art, consisting essentially of a micro-strip radiation conductor 171, a ground 172 facing thereto, a short-circuit part (short-circuit conductor) which short-circuits the radiation conductor 171 to the ground 172, and a power feed pin (feed conductor) 173 which feeds power to the radiation conductor 171. Drawings in the present specification schematically show a power feed part with an AC mark.
Since a resonance frequency of such an antenna is typically determined by a size of the radiation conductor 171, there is employed a related art method, as shown in FIG. 19, for making it dual band function possible by means of forming a slit 177 (cut-out portion) in the micro-strip radiation conductor part 171 to provide for two different resonance lengths of a lower frequency band f1 and a higher frequency band f2, whereby two resonance characteristics are produced.
A distance (spacing) between the radiation conductor 171 and the ground 172 affects the bandwidth of an antenna. Specifically, enlargement of a cubic volume sandwiched by the radiation conductor 171 and the ground 172 tends to increase the bandwidth. It should be pointed out, however, that much as an antenna can be made smaller by filling the space between the radiation conductor 171 and the ground 172 with a dielectric. The antenna made smaller in this fashion tends to result in decreasing the bandwidth.
The short-circuit part 175 is one of key features of the micro-strip inverted F antenna, and capable of reducing the radiation conductor area to about a quarter in size as compared with a micro-strip antenna devoid of the short-circuit conductor with a square shaped radiation conductor. The micro-strip antenna without the short-circuit conductor is one of the most typical type of a plane antenna.
When the power feed pin 173 is attached to a position, at which matching of an input impedance on the radiation conductor 171 to an impedance of a feed circuit (not shown) which is formed on the circuit substrate can be achieved, feeding the antenna is rendered possible.
FIG. 20 is a diagram showing an example of a micro-strip inverted F antenna disposed in a mobile wireless terminal. This is a schematic representation of parts associated with the antenna thereof, parts not associated with the configuration of the antenna being omitted.
The mobile wireless terminal is typically composed of a circuit substrate which comprises circuits required for operating of a mobile wireless terminal, a shield case for shielding the circuit substrate (not shown in the figure), and an outer frame (not shown in the figure) for protecting these parts. Installation of a built-in antenna therein may be done in several ways. In one case, a ground of the circuit substrate is used as a ground of the antenna. In another case, the shield case is used as a ground. In still another case, that is an intermediate case of these two preceding cases, the shield case makes up part of the internal portion of the antenna. In another aspect of the related art mobile wireless terminal installed with the built-in antenna, it is typical to use non-conductive material such as resin, at least, as the material of the outer frame in proximity to the antenna.
The radiation conductor 171 is made up of a sheet metal to be attached to inside of the non-conductive outer frame or mounted on a spacer disposed between a radiation conductor made of a non-ferrous metal such as a resin and a ground, whereas the short-circuit conductor and the power feed conductor are composed of a spring connector (power feed spring) of an expanding and contracting structure. The spring connector is connected mechanically and electrically to the circuit substrate by using a method such as soldering. It should be noted that the spring connector operating as the short-circuit conductor is connected to the ground of the circuit substrate, while the spring connector operating as the power feed conductor is connected to a conductor pattern formed on the circuit substrate and connected to the power feed circuit.
Furthermore, just in the possible case of a mobile wireless terminal being dropped and causing damage to its circuit substrate on strong impact, it is general practice to fix the circuit substrate with the outer frame with some degree of freedom of movement for purposes of alleviating any possible damage thereto.
With regards to the above-mentioned dual band micro-strip inverted F antenna with a slit, formation of the slit makes it substantially equivalent to the case of having two antenna elements with resonance lengths for respective frequency bands.
Nevertheless, inasmuch as these two antenna elements are in proximity to each other, the effect on mutual frequency bands, i.e., the so-called effect of mutual coupling, becomes unavoidably substantial. Namely, it is difficult for any one of the frequency bands alone to be subjected to an independent impedance adjustment.
Although the impedance adjustment can be carried out in terms of a distance adjustment between the short-circuit part 175 and the power feed pin 173, in many instances, the distance between these two parts reaching the optimum for one frequency band is different from the optimum for the other frequency. Accordingly, carrying out of independent impedance adjustment for only one of the frequency bands is not easy whatsoever. Further, the antenna occupying volume is determined by a spacing distance between the radiation conductor 171 and the ground 172 facing thereto. In the standpoint of securing antenna characteristics, it is difficult to dispose any parts necessary for a mobile wireless terminal other than an antenna in the region between the radiation conductor 171 and the ground 172.
The present invention is directed to alleviate or solve these problems. It is desired to provide a dual band built-in antenna device with antenna elements capable of conducting independent impedance adjustments for the first and second antenna elements with comparative ease, and a mobile wireless apparatus equipped therewith.
It is also desired to provide a dual band built-in antenna device with antenna elements capable of conducting adjustments regarding SAR (to be explained later) and adjustments regarding the restraint of the degradation of antenna characteristics with comparative ease, and a mobile wireless apparatus equipped therewith.
According to one embodiment of the present invention, there is provided a dual band built-in antenna device, that can be operated in a first frequency band and a second frequency band, including a ground member constituting a ground plane, a first and second inverted-L line antenna elements corresponding respectively with a first frequency band and a second frequency band. The first and second inverted-L line antenna elements are formed in a strip-line shape and configured that the two antenna elements are extended, at least initially, to different directions (directions separating from each other) from a starting position disposed in proximity to a power feed point. A separation between these two elements increases as the antenna elements extend further from the starting position. The starting point is disposed within a plane facing to the ground plane. Alternatively, the starting positions and the power feed points for the two antenna elements may be provided, respectively.
The present embodiment makes it possible to reduce the area of a radiation conductor part in each of the first and second inverted-L line antenna elements. According to the present embodiment, a smaller inverted L-shaped antenna is realized by folding a monopole antenna midway. To provide dual band compatibility (dual resonance), the possible mutual coupling effect is decreased or eliminated by constructing both antenna elements so that these elements are extended to the directions separating from each other from the starting position disposed in proximity to the power feed point that is disposed in the plane facing to the ground plane. Accordingly, each of resonance lengths of the first and the second inverted-L line antenna elements may be adjusted independently.
Formation of the line-type antenna elements contributes to increasing of the degree of freedom in disposing the first and the second antenna elements and enabling of the elements to be arranged according to a variety of purposes.
Further, in a common matching circuit, impedance matching can be conducted easily for both frequency bands.
Still further, inasmuch as the line-type antenna elements are disposed in such a way that these elements are extended to the directions separating from each other, a comparatively wide area devoid of any radiation conductor is created in the region surrounded by the antenna elements, thereby making it possible to place parts or devices other than the antenna elements thereon.